coaxial centre finder, another method

[BillTodd]

Hi Guys,

I thought you might like to see one that I made a while back.

I used a cheap laser pointer from Aldi.  It works surprisingly well.

Now that is neat  :-)

[petertha]

I used a cheap laser pointer from Aldi.  It works surprisingly well.

That's neat. If I understand - it will paint an exaggerated circle around the true center, so you move the x,y until it coincides?
- about how wide is the laser dot itself?
- can you use this for open 'holes' like a pre-drill or rotary table arbor hole, or does it need to be plugged with a point or scribe center?
- I guess varying the distance (z-axis in case of mill) kind of focuses the trace diameter?

[Arbalest]

It's on my list of items to make Baron, thanks for posting the picture!

About 2 minutes in I think:

<a href="https://www.youtube.com/watch?v=otSjut1iGGk" target="_blank">http://www.youtube.com/watch?v=otSjut1iGGk</a>

[Mosey]

This guy is shmart!
Mosey

[petertha]

Very useful demo. At 4:08 he says 'accurate to 50 microns' (~0.002"). How do you suppose that was determined? That kind of what I was wondering about width of laser trace. What is typical for these pointer gadgets? Does anyone have some links of where to get them?

The example of a centering on a vertically positioned shaft was very interesting because the laser trace hoop appears at a tilted angle around the OD, kind of magnifying the off-center deviation viewed in horizontal plane from X & Y and thus maybe could be even more accurate than the trace width? Maybe if you put a gauge pin or polished bearing ball in an existing hole that needed to be centered, that principle might work?

Definitely going to have to make one of these. Good post!

[BaronJ]

Hi Guys,

Glad you liked mine.     Thanks for looking.

Some pictures and a crude drawing below.  I cut the Aldi pointer in half so I could use just the laser bit, particularly since it embodies the batteries and on/off switch.  The laser spot is a couple of mm in diameter.  However I did notice that when I focused it, by moving the mill head up/down, there is a really tiny black spot in the very centre.  This makes it very easy to align on a scribed line or a pop mark.  A hole is even easier.  I spin mine at about 600 rpm.

One thing to watch out for is the strobe effect from the florescent lights in the workshop.  As you vary the speed of spin the laser circle breaks up into smaller and smaller segments.  This had me confused for a minute, until it dawned on me, why.

[Arbalest]

Top man, thanks for that. Angles quoted on other units folks have made seem to be between 9-11° as well. Do you know what the power of your Laser is? 5mw seems to be a popular choice at the moment.

[BaronJ]

HI Guys, Arbalest

I haven't a clue about the laser power, but I don't think it's as high as 5mw.  I do recall a yellow warning label on the side of the unit, It might have said on there.  But since I sawed it in half and threw the top bit away.      I've just had a close look at the picture with the label !  It says "<1mw" class 2.

Incidentally the fact that the two bores intersect at the bottom is deliberate.     It was done so that the laser couldn't fall out and it holds the laser firmly in place.

 

[petertha]

Sorry, I just cant leave this topic alone.
I was trying all the el-cheapo pointers I could lay my hands on in stores. The little keychain types are the right size for direct mounting in the carrier tube & self contained battery. But when I examined the dot, I was kind of dismayed at how wide & sloppy edged they were. In their defense, they are meant to hilite things on a screen, not my center finder gadget. But ideally we want as small as possible. ~1mm seems to be the smallest I could find on websites, but the store ones were a sloppy edge 2-3mm.

I went on an ebay hunt. Some quantify they dot size, others/most don't. Some talk about focal length of lenses (which I assume is related to dot size?). The mW power thing seems to be indicative of power. Low is better for us.

I'm tempted by these 'modules', but not really clear on what that infers. I'm guessing you cant just hook up DC voltage to the diode, you need the little black bits controller board? But these look interesting if I understand correctly, nice smallish size? can anyone shed more light (pun) on these things, what I should be looking for? I also hav eother ideas for other similar alignment gadgets.

http://www.ebay.ca/itm/5pcs-650nm-5mW-Red-Laser-Line-Module-Focus-Adjustable-Laser-Head-5V-good-/251688838898?pt=LH_DefaultDomain_0&hash=item3a99d2e6f2

[sco]

My interest has been piqued by these as well - we need a laser pointer expert!

Simon.

[Arbalest]

From what I've read, 5mw is perhaps a bit too powerful for our use. 1mw on the other hand may require dimming of workshop lighting to see properly. Some have said that green lasers appear brighter to our eyes than red of the same power. From this my best guess to date is something like this may be more suitable?

Link for green 1mw laser removed: Looking at it again it does say high power and that the battery runs out quickly?!

It does concern me though that some suppliers call 1mw green lasers "powerful". It makes me wonder if the quoted power output is correct?!

We do indeed need an expert to help us get through some of the possible hype. Safety is paramount.

[BaronJ]

Hi Guys,

I wouldn't worry too much about the spot size !  Yes smaller is better, but...  If you recall I mentioned that you could focus the ring down to a small spot and that I had noticed that it had a very small black centre.  As a guesstimate this black dot is 0.2 mm in diameter.

The laser diode is just that, a diode.  It's no different than an LED.  In most laser pointers there are simply three or four small cells (battery).  In the laser pointer that I used there are three 1.2 volt cells about 6 mm in diameter. No resistors, just an on/off switch.  It relies on the internal resistance of the cells to limit the amount of current that the laser diode can draw.  In the case of mine, about 0.003 ma (3 milliampere).

If you want to use a higher voltage or a battery that can supply more current, then you will need a resistor to limit the current.  There is a formula that can be used to calculate the resistor value.  You will need to know the diode forward voltage and the current that you need to pass through it.  The diode will have a maximum current rating that you must not exceed,  although from zero up to that maximum current value will determine how bright the laser will be.

Subtract the laser diode voltage from the battery supply voltage and then divide this voltage by the required current, this will give the resistance required.
Ohms Law  R = V / I

Hope this helps clarify things.

[BaronJ]

I would avoid green lasers.  Higher risk of eye damage.  1mw red is plenty bright enough.

[Arbalest]

Ok, thanks for that 1mw red then!

[petertha]

I wonder if the gadget was held in a lathe chuck/collet, could it similarly align a tail stock center? I would think any observed elliptical 'hoop' trace might actually exaggerate any lateral or height run-out & allow pretty precise setup, no? Maybe with the right angle geometry one could laser trace directly inside the tailstock MT arbor bore for that matter?